An optical coating structure is a stack of one or more thin layers of material deposited on a substrate or object in a way that alters the way in which the object reflects and transmits light. The thin layers are deposited typically to a thickness of between 10 nm to 200 nm.
For example, a quarter-wave stack reflector is a well-known building block of optical thin-film products. Such a stack generally comprises alternating layers of two or more dielectric materials with different refractive indexes, in which each layer has an optical thickness (i.e., the geometric thickness of the layer multiplied by the refractive index of the layer material) that corresponds to one-quarter of the principal wavelength of reflection. Here, the wavelength of light reflected varies with angles of incidence and reflection, thus one can observe different colors at different viewing angles; a visual effect known as iridescence.
In this manner, an optical coating structure can be constructed to accurately and selectively reflect certain wavelengths of visible light in order to impart a desired color to an object at particular angles of incidence. Unlike paints in which the color is determined by pigments or dyes that are held together with binders, with optical coating structures the transition from near total reflection to maximum transmission can take place over a very short wavelength range, enabling a precise discrimination between different wavelengths. As a result, objects coated with such structures can take on a sharp and well-defined color, albeit each color is observable over a narrow range of directions only.
It is known from JP-A-2005/153192 to provide an optical coating structure comprising a base structure that has been etched to provide a surface with a large number of small (less than 500 nm) crevices with sides that extend normal to the surface of the structure. On top of the base structure is a coating made up of two polymeric layers alternately deposited on the substrate, one of the polymeric layers having a high refractive index and the other having a low refractive index. The layers are deposited so that they imitate the uppermost surface of the base structure and as a result, each layer, including the uppermost layer of the structure, has the same profile of crevices with sides that extend normal to the surface of the structure.
The structure disclosed in JP-A-2005/153192 provides a chromogen in which the color changes with the viewing angle and provides a gentle wavelength dispersion, a deep hue and a high reflectivity. The visual effects produced by this device are, at least in part, caused by diffraction effects caused by the crevices formed in the top layer and other layers of the device.
EP 1923229 describes a security device which has an arrangement of optical elements which each comprises a stack of curved alternating layers of different refractive indices. This security device changes its appearance depending on the the type of incident illumination and provides the appearance of a single dull color over its surface when viewed under diffuse light, giving the appearance of plastic.
WO 2011/161482 discloses an optical effect structure which comprises a multilayer reflector deposited on scattering structures which are sub-micron in size. This optical effect structure provides a bright color effect with minimal iridescence.
However, it remains desirable to provide an optical coating structure capable of providing a color that is sufficiently bright and that exhibits a minimal or limited iridescence effect, i.e., so that the color remains substantially the same to an observer over a broad range of viewing angles, whilst being relatively easy, cheap and/or reliable to produce.